In the field of internal combustion engines used on merchant ships, it is known that the situation of an engine must be monitored by analyzing a lubricant circulating in this engine. Such an analysis makes it possible to detect wear or corrosion phenomena that tend to occur in an engine. In the past, the operation of the engines was relatively stabilized, and it was sufficient to inspect the quality of a lubricant periodically, at ports of call, to anticipate the maintenance operations to be performed. Today, engines are increasingly elaborate and sensitive to wear or corrosion phenomena, such that analyses must be done at sea, in particular to monitor the base number (BN) of the engine oil. This requires training staff and carrying a elaborate equipment on board, the operation of which is relatively difficult to master, even by a trained sailor. Furthermore, this increases the crew's workload.
In this context, it is known from the article “A low cost mid-infrared sensor for on line contamination monitoring of lubricating oils in marine engines” by Ben Mohammadi et al. (Optical Sensing and Detection Conference—Brussels—4/12-15/2010) to provide a system for analyzing the TBN (Total Base Number), using a sensor in which a sample of the lubricant to be studied is placed. The equipment used is elaborate, and complex to manipulate, to the point that it may be difficult to place on board a ship. Furthermore, this approach requires the user to be very knowledgeable about the measured phenomena, to the point that it is not necessarily within grasp for an ordinary seaman. This equipment uses a model that is only suitable for a single type of oil. It therefore becomes very difficult to incorporate it into a process where the liquids used change continuously.
WO-A-03/073075 discloses a method for analyzing the base number of a lubricant during which a measurement, done on a sample of the lubricant to be inspected, is compared to measurements done on reference samples. The method considered in this document is based on the use of the absorbance of the lubricant, which is relatively imprecise, since it is sensitive to noise from the collected information. Indeed, the erratic or “noise” values risk being considered as relevant as the significant values measured. This technique requires a “blank” before each measurement, which is restrictive and leads to lost time. Furthermore, it is necessary to clean the sensor after each measurement, since residue from the previous sample would alter the current measurement. Lastly, the model obtained with this method is valid for a threshold lubricant type and is highly dependent on the reading precision of the absorbance.
It is also known from WO-A-2013/186338 to determine an overall basicity index of a lubricant through FTIR analysis of an absorption spectrum. This method is complex to implement.
In general, comparable problems arise when the basicity index of a liquid body must be determined.